High-purity pure water (so-called ultrapure water) from which impurities have been removed to a high degree and which are used in semiconductor production, biotechnology, and other applications has reached a level of extremely high purity as a result of recent advances in ion exchange resin treatments or distillation techniques. At present, the electrical resistivity of high-purity pure water is close to 18 M.OMEGA./cm, i.e., a state in which almost no free ions are present in the water. However, even pure water having such a high degree of purity contains the dead cells of microorganisms, such as bacteria, and nonionic substances. These remaining impurities in the water are impossible to remove by an ion exchange method or distillation method, and contaminants such as these are an obstacle to further improving water purity. The amount of impurities which ultrapure water can contain is gradually becoming more restrictive, and present-day ultrapure water must contain less than 10 contaminant particles per milliliter (ml) with a size of 0.07 .mu.m or larger.
On the other hand, ozone is attracting attention as a powerful and clean oxidizing agent. The use of ozone, particularly for water treatment, is increasing since treatment with ozone is advantageous. For example, no residual substances are left in the treated water unlike conventionally employed chlorine-containing oxidizing agents. The lack of a residual substances is because the product of ozone decomposition is oxygen, and the decomposition rate of ozone is so high that ozone itself does not remain in the treated water. Hence, there are no problems of secondary pollution.
To improve the purity of ultrapure water, ozone treatment is performed preferably in combination with an ion exchange method or a distillation method. In conducting ozone treatment of water for producing ultrapure water having an improved purity, contamination of the water due to the ozone gas itself supplied for the treatment should be avoided.
For evolving gases containing ozone which is a useful oxidizing agent as described above, an electrical discharge method and an electrolytic method have been primarily employed conventionally. However, the electrolytic method is used typically because of its advantages of product purity and operational efficiency.
The electrolytic method, when carried out using a fluororesin-type ion-exchange membrane as a solid electrolyte, is capable of producing an ozone-containing gas having a far higher purity and ozone concentration as compared with the electrical discharge method. However, it was recently found that a slight amount of fluorine-containing substances is present in the ozone-containing gas generated by such an electrolytic method. This phenomenon had not been observed until recently and, hence, no method for removing such contaminants is presently known.